1. Field of the Invention
The present invention relates to an antenna for use in the microwave and millimeter-wave range, and more particularly to a dielectric antenna for radiating an electromagnetic wave from a dielectric.
2. Description of the Background Art
Dielectric antennas loaded with a dielectric block placed over a feed circuit, which includes a microstrip line, a waveguide, etc., have been widely used in the art for radio communications in the microwave and millimeter-wave range (see Japanese Laid-Open Patent Publication Nos. 2000-209022 and 2000-278030). Such dielectric antennas are called “waveguide-fed dielectric antennas”.
FIG. 62 is an exploded perspective view illustrating a conventional waveguide-fed dielectric antenna. Referring to FIG. 62, the conventional dielectric antenna includes a lower conductor plate 101, an upper conductor plate 102 and a loading dielectric block 103 having a cylindrical shape. The lower conductor plate 101 includes a feed port 104, a first waveguide groove 105 and a depressed portion 106. The upper conductor plate 102 includes a second waveguide groove 107 and an aperture 108.
The upper surface of the lower conductor plate 101 and the lower surface of the upper conductor plate 102 are attached to each other. As the plates are attached to each other, the first waveguide groove 105 and the second waveguide groove 107 together form a waveguide.
The loading dielectric block 103 is bonded to the upper conductor plate 102 over the aperture 108. Placing a dielectric block on a substrate is termed “loading with a dielectric block”.
An electromagnetic wave inputted to the feed port 104 travels through the inside of the waveguide, leaks through the aperture 108, and is fed to the loading dielectric block 103 and radiated therefrom. In this process, there appear two types of electromagnetic waves. The first is an electromagnetic wave traveling through the inside of the loading dielectric block 103. The second is an electromagnetic wave traveling along the surface of the loading dielectric block 103 (“surface wave”). The loading dielectric block 103 has such a size that the two types of electromagnetic waves are in phase with each other at the upper surface of the loading dielectric block 103. As the two types of electromagnetic waves are brought in phase with each other at the upper surface of the loading dielectric block 103, it is possible to provide an antenna with a high gain.
For example, consider a conventional dielectric antenna having a structure as illustrated in FIG. 62. In the conventional dielectric antenna, the lower conductor plate 101 is made of aluminum, and has a size of 100 mm×100 mm and a thickness of 3 mm. The upper conductor plate 102 is made of aluminum, and has a size of 100 mm×100 mm and a thickness of 2.5 mm. The waveguide formed by the first waveguide groove 105 and the second waveguide groove 107 has a size of 3.76 mm×1.88 mm. The aperture 108 has a size of 2.8 mm×2.8 mm. The loading dielectric block 103 is made of polypropylene (relative dielectric constant: 2.26), the diameter φ thereof is 6 mm, and the height L thereof is 7 mm.
FIG. 63 is a graph showing the radiation pattern along the xz plane (electric field plane) for a dielectric antenna as described above. In FIG. 63, the vertical axis represents the gain of the antenna. The horizontal axis represents the angle in the xz plane with respect to the center of the loading dielectric. As shown in FIG. 63, the dielectric antenna exhibits a high gain in the range of about ±30 degrees.
An antenna using a post-wall waveguide is described in “Reflection-Canceling Slot Pair Array with Cosecant Radiation Pattern Using a Millimeter-Wave Post-Wall Waveguide” by Jiro Hirokawa, 2000 IEICE Communications Society Conference (2000), B-1-61, p. 61, and “Slot Antenna with a Sector Beam on a Millimeter-Wave Post-Wall Waveguide” by Jiro Hirokawa and one other, 2000 IEICE General Conference (2000), B-1-133, p. 133.
However, as shown in FIG. 63, the conventional dielectric antenna has a high gain only in the range of about ±30 degrees with respect to the center of the loading dielectric block 103. Therefore, the conventional dielectric antenna has a small beam width. Thus, the conventional dielectric antenna has a narrow coverage. In a frequency range where the space attenuation is substantial, such as a millimeter-wave range, for example, it is of course necessary to use an antenna with a high gain, and the antenna may also be required to have a wide coverage for some applications. Thus, it is in some cases necessary to use an antenna with a high gain and a large primary beam width.
Moreover, the conventional dielectric antenna as illustrated in FIG. 62 uses a metal waveguide including two metal plates attached together as the feed circuit, whereby the dielectric antenna is large and heavy. Thus, the conventional dielectric antenna requires high machining cost. The antenna as a whole can be downsized by filling the inside of the waveguide with a dielectric. However, it requires a difficult operation to evenly fill the inside of the waveguide with a dielectric. Therefore, the dielectric filling has not been a practical option.